1. Field of the Invention
The present invention relates generally to resistance measuring or indicating devices, and more particularly, to devices for indicating variations in the electrical resistance of the human body.
2. Description of the Prior Art
Devices for measuring or indicating the electrical resistance of a human body are known in the art and have found usage in psychological counselling for detecting stress and in other biofeedback applications. U.S. Pat. No. 3,290,589 discloses an electrical resistance measuring or indicating device comprising a Wheatstone bridge network having on one side thereof a first resistance arm connected to a second resistance arm, and on the other side thereof a first voltage arm connected to a second voltage arm. Between the junction of the first and second resistance arms and the junction of the first and second voltage arms there is a transistorized amplifier circuit and moving coil meter responsive to changes in balance of the bridge network, so as to indicate the resistance, or variations in resistance, of a subject such as a body connected to the network across one of the resistance arms.
In a preferred form of the aforementioned prior art device, a first variable potentiometer is associated with one of the voltage arms for controlling the range over which the device can operate to indicate variations in resistance and a second variable potentiometer is connected between the junction of the first and second resistance arms and the amplifier circuit. When the bridge network becomes unbalanced by the application of a subject thereto, balance can be restored only by the manual adjustment of the first potentiometer so that the meter will again respond to very small changes in the resistance of said subject.
Although this prior art device is useful, the need for manual adjustment of the first potentiometer means in order to balance the bridge network represents a shortcoming. When a subject applied to the bridge network, the network can become unbalanced frequently as it responds to changes in the subject's resistance. In the aforementioned prior art device, frequent manual intervention can therefore be required in order to perform adjustments of the first potentiometer to balance the bridge. This can be distracting to the person operating the device. Moreover, in view of the possibility of error which is attendant to operations requiring manual intervention, the potentiometer can be over- or under-adjusted when the operator attempts to restore balance to the bridge network. Over- or under-compensation requires further adjustment of the potentiometer to balance the device. Another shortcoming of this prior art device is related to the accuracy of its resistance indications. In the preferred embodiment of the prior art device, a range control potentiometer knob is provided to facilitate manual adjustment of the resistance range. The control knob points to positions on a linear scale calibrated in values related to subject resistance. The reading of the value on the linear scale which is pointed to by the potentiometer control knob requires operator interpretation and judgement as to the exact position of the knob on the scale. The measurements obtainable from the device are thereforesubject to a degree of human error.
It is a principal object of this invention to provide an improved device for measuring and indicating changes in resitance of a living body.
It is a specific object of the present invention to provide a device for measuring and indicating changes in resistance of a living body in which adjustment and balancing of the bridge network of the device, as it responds to resistance changes in the living body, are performed automatically by an electronic circuit instead of manually by the adjustment of a potentiometer.
It is another object of the present invention to provide a device which calculates and digitally displays the range control subject resistance value instead of requiring interpolation of a scale to discern the resistance value.
Through the fulfillment of the aforementioned objects, a final object of the invention is to provide a device for measuring and indicating changes in resistance in a living body which is easier to use and more accurate than prior art devices of the type described above.